1. Field of the Inventive Concept
Exemplary embodiments of the inventive concept relate to a high speed linear differential amplifier, and more particularly, to a high speed linear differential amplifier having a gain adjusted by an applied control voltage.
2. Description of Related Art
A differential amplifier is a circuit that detects and amplifies a voltage difference between input signals input to two input terminals and that outputs an amplified voltage. The ratio of a change in voltage difference between output signals to a change in voltage difference between input signals is called the “gain” of the differential amplifier. Differential amplifiers with a large gain widely used, but a differential amplifier with a characteristic of being capable of securing a wide range in which an output signal linearly changes with respect to a change of a voltage difference between input signals rather than the gain is also required according to necessity.
FIG. 1 is a graph illustrating the range of the gain (the ratio of the change of an output voltage with respect to a change in input voltage) of a differential amplifier. Hereinafter, it is assumed that two input signals are a pair of differential input signals which are inverted relative to each other. As illustrated in FIG. 1, when the gain of the differential amplifier is large (g1), the change of an output voltage Vout with respect to a change of an input voltage Vin is large, but range the voltage range within which the output voltage Vout changes linearly with respect to a change in the input voltage Vin is small. This is because a possible voltage level of the output voltage Vout of the differential amplifier is bounded by a maximum output voltage OUTmax and a minimum output voltage OUTmin. On the other hand, when the gain g3 of the differential amplifier is smaller, the change of the output voltage Vout with respect to the change of the input voltage Vin is small, but the range in which the output voltage Vout linearly changes with respect to a change in the input voltage Vin is larger. Therefore, the output voltage Vout linearly changes as the voltage level of the input signal changes in a wide range. However, since the voltage level of the input voltage Vin also has minimum and maximum voltage values, when the gain g3 of the differential amplifier is small, the swing width of the output voltage Vout to the change of the input voltage does not reach the maximum output voltage OUTmax and the minimum output voltage OUTmin. Thus, as illustrated in FIG. 1, when the gain g3 of the differential amplifier is too small, the output voltage Vout stays in a voltage range between the maximum gain output voltage g3max and the minimum gain output voltage g3min because the input is not amplified up to the maximum output voltage OUTmax and the minimum output voltage OUTmin for maximum and minimum input voltages INmax and INmin, respectively. Therefore, the swing width of the output voltage Vout of the differential amplifier is reduced when the gain g3 of the differential amplifier is too small.
When the differential amplifier has a gain g2 between the gain g1 and the gain g3, the output voltage Vout has a maximum swing width while linearly changing with respect to the change of the input voltage Vin of the differential amplifier. Therefore, it is important that the differential amplifier can linearly change the output voltage Vout in a range between the minimum and maximum voltages of the input signal and has the gain g2 which provides the maximum output voltage swing width.
The linear differential amplifier in which the output signal linearly changes can be used for impedance conversion, various operations, equalizers, pre-amplification of a multi-level signal, and common mode removal of the input signal since the output signal linearly changes in the entire swing width range of the input signal. Thus, since the linear differential amplifier outputs the output signal that maintains the waveform of the input signal, the linear differential amplifier is used in circuits that detect an input signal at multiple levels (i.e., not only a high and low level).
A linear differential amplifier is frequently used as an input/output circuit in circuits that detect the input signal at multiple levels, and can function as a high speed input/output circuit since the linear differential amplifier linearly amplifies changes of the input signal in the whole range of the output signal as described above.
In order to configure the gain of a linear differential amplifier, a transistor or a resistor is typically adjusted. However, such amplitude (gain) adjustment circuit elements may cause the gain of the differential amplifier to vary undesirably due to a changes in process, temperature, and swing width of the input signal. When the gain changes, the output signal of the differential amplifier may not linearly change over an entire swing width range of the input signal.